So I'm currently taking summer school for grade 12 English. Lately I've been hearing that some universities do not accept summer school courses, or that summer school is looked down upon. Do any of you know if summer school will affect my chances of getting into Mechanical Engineering?

No it will not affect your chances here at McMaster; all you need is a passing grade. I wouldn't see it affecting you anywhere else either unless you're retaking this course due to a failure, or you've done a significant amount of summer school or night school for major courses. These things would only affect you if you're applying to really competitive engineering programs such as Waterloo or U of T as they have supplementary applications.

The only thing summer courses are good for is to bump up your final grade so you get a better entrance scholarship, but like Grim said, it doesn't affect you getting accepted, as that's decided before June (as in before you finish High School).

Well, as they do not tell students what their algorithms are for acceptance, all anyone can tell you is what is publicly available - which for most programs at Mac, is essentially them ordering the applicants from highest to lowest GPA and then taking the top x number of applicants. Of course, programs with a supplemental application are different, but again, they do not tell students what their exact criteria are.

If Mac looks at different high schools differently, or different types of courses differently, they do not admit to doing so.

For what it's worth (and it should be worth very little), I took grade 12 English in summer school (summer before grade 12, which I assume is what you are doing), and I was accepted into Mac.

McMaster doesnt care about what type of course it is (aka summer school, private school or in regular high school). All that matters are your grades.